In a manner similar to an electric light bulb that gives off heat when it is powered on, electronics such as computer equipment dissipate electrical power as heat. In many cases, to prevent the electronics from overheating, that heat must be removed by a cooling system, typically using a cooling fluid such as air or water that is passed through the electronics. The current invention applies to all types of cooling fluids and many types of electronic equipment, but for specificity, consider the example of a liquid-cooled computer. Referring to the traditional system 100 shown in FIG. 1, electrical power is consumed by liquid-cooled computers 102 in a machine-room 104. The computers' electrical power is dissipated as an arbitrary number of power consumptions (or heat loads) PA, PB, . . . , PX, which are removed to the outside world in several steps.
First, the power consumption of the electronics (or aggregate machine-room heat load)P0≡PA+PB+ . . . +PX  (1)is transferred from the computers to a liquid coolant 106, which enters each computer at a cold temperature T0, and flows under pressure created by pump 108 in a closed loop of liquid-coolant pipes. The pump 108 creates an additional power consumption (or heat load) P1.
Second, in chiller 110, the combined heat load P0+P1 is transferred from the liquid coolant 106 to a refrigerant 112, which flows in a closed loop of refrigerant pipes, wherein evaporation of the refrigerant 112 occurs during its absorption of heat load P0+P1, and compression of the refrigerant occurs in a compressor 114. The compressor 114 creates an additional heat load α(P0+P1) that is proportional to the incident heat load P0+P1. The proportionality factor α is a characteristic of the compressor 114. The temperature T0 entering computers 102 is maintained by feedback circuitry comprising a temperature-measurement device 116 and a processing device 118 that compares the measured value T0 to a user-defined set-point temperature (T0)Set, and commands the chiller 110 to drive the difference T0−(T0)Set to zero by modulation of the compressor 114.
Third, heat load (1+α)(P0+P1) is transferred from the refrigerant 112 to condenser water 120, which flows under pressure created by pump 122 in a closed loop of condenser-water pipes, with the transfer of heat (1+α)(P0+P1) causing condensation of the refrigerant 112. The pump 122 creates an additional heat load P2.
Fourth, by means of a cooling tower 124, which typically contains an air mover 126 that produces an additional heat load P3, heat load (1+α)(P0+P1)+P2+P3 is transferred from the condenser water 120 to the outside air.
The “useful” electrical power consumed in the machine room 104 by computers 102 is P0, whereas the “overhead” power consumed by the cooling equipment in system 100 is, by inspection of FIG. 1,PCool(100)≡P1+α(P0+P1)+P2+P3.  (2)
Thus, the total electrical power consumed by system 100 isPTotal(100)=P0+PCool(100).  (3)
In equations (2) and (3), superscript “(100)” indicates that the symbols apply to system 100.
The dominant term on the right-hand side of equation (2) is the compressor power α(P0+P1), where the chiller-overhead fraction α is typically 0.10 to 0.20. Consequently, to save some or all of compressor power a (P0+p), the concept of “free cooling” has been developed in prior art, as described, for example, in “Free Cooling Using Water Economizers”, by Susanna Hanson and Jeanne Harshaw, TRANE Engineers Newsletter, Volume 37-3, September 2008, which is incorporated herein in its entirety by reference. It is worth noting that in the Unites States, the overhead fraction α is often expressed in kW/ton, where a ton of cooling is 3.517 kW. Thus, for example, 0.53 kW/ton corresponds to the dimensionless value α=0.53/3.517=0.15.
Referring to FIG. 2, a typical “free cooling” system 200 is to some extent similar to the traditional system 100; namely, callouts 202 through 226 of system 200, shown on FIG. 2, are exactly analogous, respectively, to callouts 102 through 126 of system 100, shown on FIG. 1. However, system 200 is distinguished from system 100 by the addition of a free-cooling heat exchanger 230 to the loop of liquid-coolant loop 206, which allows the liquid coolant 206 to reject some of its heat load P0+P1 to free-cooling water 232. Letβ≡Fraction of heat load P0+P1 that coolant 206 rejects to free-cooling water 232 via the free-cooling heat exchanger 230.  (4)
Free-cooling water 232 flows under pressure created by a pump 234 in a closed loop of pipes. The pump 234 creates an additional heat load P6. By means of a cooling tower 238, which typically contains an air mover 240 that produces additional heat load P7, a heat load β(P0+P1)+P6+P7 is transferred from the free-cooling water 232 to outside air.
System 200 is further distinguished from system 100 by the addition of feedback circuitry comprising a device 242 to measure temperature T3 and to communicate this measurement to processing device 218. System 200 is further distinguished by the addition of electrical feedback from processing device 218 to pump 234 to enable speed modulation of the pump is cases where T3<T0, so that the liquid coolant does not become too cold, and also to enable powering off the pump in cases where T3>T1, to prevent undesired heating of the liquid coolant as it passes through the free-cooling heat exchanger 230.
Because of the heat-load rejection β(P0+P1) from liquid coolant 206 by means of heat exchanger 230, the amount of remaining heat to be rejected to the refrigerant 212 in system 200 is (1−β)(P0+P1). Thus, the incident heat load on the compressor 214 in system 200 is a factor of 1−β smaller than that on the compressor 114 in system 100. Because we assume the compressors 114 and 214 to be otherwise identical, the power consumption of compressor 214 in system 200 is likewise reduced by the factor 1−β compared to compressor 114 in system 100. That is, system 200's compressor 214 consumes only (1−β)α(P0+P1).
In comparing systems 100 and 200 to assess how much power is saved by “free cooling”, the useful electrical power P0 consumed in the machine room 204 of system 200 is naturally assumed to be the same as that consumed in the machine room 104 of system 100. This assumption merely states that, to make a fair comparison, the computers 202 in system 200 are identical to the computers 102 in system 100, and are performing identical computations.
The “overhead” power consumed by cooling equipment in system 200 isPCool(200)≡P1+(1−β)α(P0+P1)+P4+P5+P6+P7.  (5)Comparing equations (2) and (5) yields the power-saving advantage of system 200 over system 100:ΔP≡PCool(100)−PCool(200)=βα(P0+P1)+(P2+P3)−(P4+P5+P6+P7)  (6)
The power consumed by pumps (P1, P4, P6) and air movers (P5, P7) is typically small compared to that consumed by the refrigerant-loop compressor, so the first term on the right-hand side of equation (6), βα(P0+P1), is the dominant teem. Moreover, if the pumps 122, 222, and 234 as well as the cooling-tower air movers 126, 226, and 240 are controlled so that power consumed is proportional to incident heat load, thenP4=(1−β)P2;P6=βP2  (7)andP5=(1−↑)P3;P7=βP3.  (8)Assuming this type of control, by combination of equations (7) and (8),P4+P5+P6+P7=P2+P3,  (9)whence, substituting equation (9) into equation (6), the second the third terms on the right-hand side of equation (6) disappear, and the power saving from free cooling becomes simplyΔP≡PCool(100)−PCool(200)=βα(P0+P1).  (10)
Equation (10) implies that to maximize the amount of saved power ΔP, β=1 is desired, whereby the entire incident heat load P0+P1 on free-cooling heat exchanger 230 is rejected thereby. In such a scenario, the chiller 210, pump 222, and cooler tower 126 may be turned off. In fact, if β=1 can be achieved at all times, then the chiller 212, pump 222, and cooling tower 224 are superfluous and need not be purchased. This is the most aggressive objective of the free-cooling paradigm.
Still referring to FIG. 2, this aggressive objective can typically only be achieved, unfortunately, by permitting the coolant 206 that enters computers 202 to have a high temperature T0. In general, from energy arguments, the relation between β and coolant temperatures is
                    β        =                                                            T                1                            -                              T                2                                                                    T                1                            -                              T                0                                              .                                    (        11        )            
Consequently, if the chiller 210 is turned off or absent, then β=1, so, according to equation (11), T2=T0. But T2 is weather dependent, because the temperature T3 of water returning from the cooling tower 238 depends on the wet-bulb temperature TWB of ambient outside air, which depends on geographical location and season. The wet bulb temperature TWB currently never exceeds 31° C. anywhere on earth, but may be higher in the future due to climate change, as reported by Steven C. Sherwood and Matthew Huber in “An adaptability limit to climate change due to heat stress”, Proceedings of the National Academy of Sciences, May 2010, (0913352107), which is included herein in its entirety by reference. Temperature T3 exceeds TWB by an amount ΔTCT, sometimes called the “cooling-tower approach temperature”, which is a function of several variables (see the aforementioned TRANE Engineers Newsletter) but typically in the range of 1 to 5° C. Thus,T3=TWB+ΔTCT.  (12)
Moreover, temperature T2 exceeds T3 by an amount ΔTHX, sometimes called the “heat-exchanger approach temperature”, which is typically in the range of 1 to 2° C. Thus,T2=TWB+ΔTCT+ΔTHX.  (13)
Consequently, to achieve year-round free cooling anywhere on earth under current climate conditions, the water-inlet temperature T0 to computers 202 may need to be as high as(T0)max≡(TWB)max+(ΔTCT)max+(ΔTHX)max=31+5+2=38° C.  (14)In contrast,(T0)min≡16° C.  (15)is just warm enough to avoid condensation of air-borne moisture for “Class 1” machine-room conditions, as specified by the American Society of Heating, Air-Conditioning, and Refrigeration Engineers (ASHRAE) in “Thermal Guidelines for Data Processing Environments, 2nd edition”, ISBN 978-1-933742-46-5, which in included herein in its entirety by reference. Specifically, ASHRAE's “Recommended” Class 1 envelope in psychrometric space is bounded above by a 15° C. dew-point, which implies, with a 1° C. margin of safety, that T0=16° C. is the minimum safe temperature at which air-borne water is guaranteed not to condense inside the computers 202.
Less aggressive than equation (14), a more typical free-cooling option is to choose a moderate set-point value of T0, and to retain the chiller 210, pump 222, and cooling tower 224 so that, when weather precludes the free-cooled temperature T2 from reaching the desired set-point T2=(T0)Set, the chiller can make up the difference. Even in such systems, with a chiller present as in system 200, users of computers and other electronic equipment urge manufacturers to design their equipment to allow high inlet temperature T0 so that, despite adverse weather conditions, the fraction β of the heat load removed by the heat exchanger 230 remains high, and thus the power-saving ΔP, given by equation (10), remains large. This strategy says that maximizing the amount of free cooling is always better—even if it means higher water inlet temperature T0 to the computers 202.
The current invention calls this strategy into question, purely on the basis of power savings. It explains why maximizing the amount of free cooling, regardless of T0, does not necessarily save energy, but may actually waste energy. Based on this insight, the invention provides, for a system like system 200, an innovative method and apparatus to determine the value of T0 that actually provides the greatest conservation of energy. Depending on the weather-related temperature 7; and the computational state C, the power-optimal solution may be for the chiller 310 to provide some or even all of the cooling, despite the presence of the free-cooling heat exchanger 330.